Is there a way to limit a query in discrete groups? For example, let's say I have this query below.
| col1 | col2 |
---------------
| 1 | A |
| 1 | B |
| 2 | C |
| 2 | D |
| 3 | E |
| 3 | F |
I want the limit on this query to be 5 rows. However, I only want it to show discrete complete groups based on the first column. So that means I don't want to show (3, E) since (3, F) would be cut off. So it would only show the first 4 rows.
Is there a way to write this dynamic logic into a MySQL query?
Count rows in a subquery:
select col1, col2
from mytable m1
where (select count(*) from mytable m2 where m2.col1 <= m1.col1) <= 5
order by col1, col2;
Related
Is it possible to use LIMIT based on another column inside a subquery in MySQL? Here is a working query of what I mean.
SELECT id, name,
(SELECT AVG(value) FROM t2 WHERE t1id = t1.id ORDER BY value DESC LIMIT 4) as average
FROM t1
However I'd like to replace the "4" to a field inside t1.
Something like this where table t1 has fields id, name, size:
SELECT id, name,
(SELECT AVG(value) FROM t2 WHERE t1id = t1.id ORDER BY value DESC LIMIT t1.size) as average
FROM t1
I could join t1 and t2, but I'm not sure that works for this. Does it?
Edit:
Here's some sample data to show what I mean:
Table t1
| id | name | Size |
|----|------|------|
| 1 | Bob | 4 |
| 2 | Joe | 3 |
| 3 | Sam | 4 |
Table t2
| t1id | value |
|------|-------|
| 1 | 16 |
| 1 | 14 |
| 1 | 12 |
| 1 | 10 |
| 1 | 8 |
| 2 | 10 |
| 2 | 8 |
| 2 | 6 |
| 2 | 4 |
| 3 | 20 |
| 3 | 15 |
| 3 | 10 |
| 3 | 5 |
| 3 | 2 |
Expected result:
| id | name | avg |
|----|------|------|
| 1 | Bob | 13 |
| 2 | Joe | 8 |
| 3 | Sam | 12.5 |
Notice that the average is the average of only the top t1.size values. For example the average for Bob is 13 and not 12 (based on 4 values and not 5) and the average for Joe is 8 and not 7 (based on 3 values and not 4).
In MySQL, you have little choice other than LEFT JOIN and aggregation:
SELECT t1.id, t1.name, AVG(t2.value) as average
FROM t1 LEFT JOIN
(SELECT t2.*,
ROW_NUMBER() OVER (PARTITION BY t1id ORDER BY VALUE desc) as seqnum
FROM t2
) t2
on t2.t1id = t1.id AND seqnum <= t1.size
GROUP BY t1.id, t1.name;
Here is a db<>fiddle.
No, you cannot use a column reference in a LIMIT clause.
https://dev.mysql.com/doc/refman/8.0/en/select.html has detailed documentation about MySQL's SELECT statement including all its clauses.
It says:
The LIMIT clause can be used to constrain the number of rows returned by the SELECT statement. LIMIT takes one or two numeric arguments, which must both be nonnegative integer constants, with these exceptions:
Within prepared statements, LIMIT parameters can be specified using ? placeholder markers.
Within stored programs, LIMIT parameters can be specified using integer-valued routine parameters or local variables.
Expressions, including subqueries, are not mentioned as legal argument in the LIMIT clause.
A simple solution would be to do your task in two queries: the first to get the size and then use that value as a constant value in the second query that includes the LIMIT.
Not every task needs to be done in a single SQL statement.
I have a table that has an autoincremented numeric primary. I'm trying to get a count of rows that match a condition grouped by increments of their primary key. Given the data:
| id | value |
|----|-------|
| 1 | a |
| 2 | b |
| 3 | a |
| 4 | a |
| 5 | b |
| 6 | a |
| 7 | b |
| 8 | a |
| 9 | b |
| 10 | b |
| 11 | a |
| 12 | b |
If I wanted to know how many rows matched value = 'a' for every five rows, the result should be:
| count(0) |
|----------|
| 3 |
| 2 |
| 1 |
I can nest a series of subqueries in the SELECT statement, like such:
SELECT (SELECT count(0)
FROM table
WHERE value = 'a'
AND id > 0
AND id <= 5) AS `1-5`,
(SELECT count(0)
FROM table
WHERE value = 'a'
AND id > 5
AND id <=10) AS `6-10`,
...
But is there a way to do this with a GROUP BY statement or something similar where I don't have to manually write out the increments? If not, is there a more time efficient method than a series of subqueries in the SELECT statement as in the above example?
You could divide the ID by 5 and then ceil the result:
SELECT CONCAT((CEIL(id / 5.0) - 1) * 5, '-', CEIL(id / 5.0) * 5), COUNT(*)
FROM mytable
WHERE value = 'a'
GROUP BY CEIL(id / 5.0)
The following aggregated query should do the trick :
SELECT CEIL(id/5), COUNT(*)
FROM table
WHERE value = 'a'
GROUP BY CEIL(id/5)
Is there a way to sum rows only when they meet certain condition?, If they dont they must be copied to the new table. For example, if i have this table
| id | A | B |
--------------
| 1 | a | 2 |
| 1 | b | 4 |
| 1 | c | 1 |
| 2 | a | 4 |
| 3 | a | 1 |
| 3 | b | 5 |
I want an output like this
| id | A | B |
--------------
| 1 | a,b | 6 |
| 1 | c | 1 |
| 2 | a | 4 |
| 3 | a,b | 6 |
It will only sum if column 'A' is 'a' or 'b', it will just copy the value if its 'c'
You can do this in two different Select queries. In first Select query, just consider the cases where A has either values 'a' or 'b'. In the second Select query, consider the rest of the values (A NOT IN ('a','b'))
Use UNION ALL to combine the results into a Derived Table.
Order the Derived table result-set in ascending order by id.
We use aggregation functions like Group_concat() and Sum() to get comma separated string (for a and b), and sum of the B values, respectively.
Try the following:
SELECT dt.*
FROM
(
SELECT id,
GROUP_CONCAT(DISTINCT A) AS A,
SUM(B) AS B
FROM your_table
WHERE A IN ('a','b')
GROUP BY id
UNION ALL
SELECT id,
A,
B
FROM your_table
WHERE A NOT IN ('a', 'b')
GROUP BY id, A, B
) AS dt
ORDER BY dt.id ASC
Your example does not show all possible cases.
But I think that probably you don't need to overcomplicate solution with UNION
http://sqlfiddle.com/#!9/d473d3/6
SELECT id,
GROUP_CONCAT(A),
SUM(B)
FROM abc
GROUP BY CONCAT(id, IF(A IN ('a','b'),'$',A))
This seems to be a convoluted problem, but I'll try my best to articulate the idea and illustrate a scenario. Essentially I have two tables that need to be combined and returned as the result set for a single query. One table needs to be merged into the other in a specific order.
Say table one is called Articles and table two is called Features. Both tables have an ID field with unique numbers. Articles has a date field which will be used to initially sort its records in descending order. The Features table has a Delta field which be used initially to sort its records. Some of the records in the Features table are placeholders and are not meant to be included in the final set. Their only purpose is to affect the sort order. Each record has a unique value in the Delta field, from 1 - X which will be used to sort these records. Another field called Skip has a value of 1 if it should be eliminated when merging the two tables together. Again, the only purpose to the skipped records is to take up space during the initial sort on the Features table. Even though they are unnecessary, they exist and can't be deleted.
The tricky part is that when the results from both tables are merged, any non-skipped records from the Features table need to be inserted into the results from the Articles table in the exact order they appears in the Features table.
So lets say I have 6 records in the Features table, A - F and the order field ranges from 1 - 6. Records A,B,D,E all have a value of 1 in the Skip field. That means I'm only interested in records C and F both of which need to be inserted into the final record set in positions 3 and 6 respectively.
The records may look something like this for the Articles table:
+----+------------+
| id | date |
+----+------------+
| 1 | 9999999999 |
+----+------------+
| 2 | 9999999998 |
+----+------------+
| 3 | 9999999997 |
+----+------------+
| 4 | 9999999996 |
+----+------------+
| 5 | 9999999995 |
+----+------------+
| 6 | 9999999994 |
+----+------------+
| 7 | 9999999993 |
+----+------------+
| 8 | 9999999992 |
+----+------------+
| 9 | 9999999991 |
+----+------------+
| 10 | 9999999990 |
+----+------------+
The Features table may look something like this:
+----+------+-------+------+
| id | name | delta | skip |
+----+------+-------+------+
| 11 | A | 1 | 1 |
+----+------+-------+------+
| 12 | B | 2 | 1 |
+----+------+-------+------+
| 13 | C | 3 | 0 |
+----+------+-------+------+
| 14 | D | 4 | 1 |
+----+------+-------+------+
| 15 | E | 5 | 1 |
+----+------+-------+------+
| 16 | F | 6 | 0 |
+----+------+-------+------+
The results would look something like this (not including any additional fields that might be needed to achieve my goal):
+----+
| id |
+----+
| 1 |
+----+
| 2 |
+----+
| 13 | (record from the Features table in the third position)
+----+
| 3 |
+----+
| 4 |
+----+
| 16 | (record from the Features table in the sixth position)
+----+
| 5 |
+----+
| 6 |
+----+
| 7 |
+----+
| 8 |
+----+
| 9 |
+----+
| 10 |
+----+
Hope my explanation makes sense. Any ideas?
Thanks,
Howie
I assume that there is a mistake in your example - record id=16 is sixth row in Features table, so should be after id=5 in results, not before.
Try the blelow query. Here is SQLFiddle.
select id from (
select `date`, null delta, id
from Articles
union all
select a.`date`, f.delta, f.id
from (
select (#x:=#x+1) rn, a.*
from Articles a, (select #x:=0) z
order by a.`date` desc
) a
join (
select (#y:=#y+1) rn, f.id, f.delta, f.skip
from Features f, (select #y:=0) z
order by f.delta
) f
on a.rn = f.rn
where f.skip <> 1
order by `date` desc, isnull( delta ), delta
) merge
Looks like this example in SQL Fiddle did it for me.
SELECT id, sort_order FROM (
SELECT `date`, NULL delta, id, (#a_count:=#a_count+1) sort_order
FROM Articles a_main, (SELECT #a_count:=-1) z
UNION ALL
SELECT a.`date`, f.delta, f.id, f.weighted_rn
FROM (
SELECT (#x:=#x+1) rn, a.*
FROM Articles a, (SELECT #x:=-1) z
ORDER BY a.`date` DESC
) a
JOIN (
SELECT (#y:=#y+1) rn, TRUNCATE((f.delta - #y - (1/#y)),2) AS weighted_rn, f.id, f.delta, f.skip
FROM Features f, (SELECT #y:=-1) z
WHERE f.skip <> 1
ORDER BY f.delta
) f
ON a.rn = f.rn
ORDER BY sort_order
) merge
Thanks to Kordirko for the framework.
I have these data in database:
+----+------------+---------+
| id | col1 | col2 |
+----+------------+---------+
| 1 | 3 | 2 |
| 2 | 2 | 3 |
| 3 | 4 | 5 |
+----+------------+---------+
I am trying to do a query, that will give me a count of rows, in which are the same numbers.
I know the value one of them (the numbers).
In this case, the same numbers are 2 and 3 (in the columns col1 and col2) - I am trying to get from database number 2 (two rows). I have available always one of two numbers - in this case number 3.
Is possible to do this query?
Thanks for help.
For the very narrow scenario you specified, I would try this:
select count(*)
from myTable mt1
inner join myTable mt2 on (mt1.col1 = mt2.col2 AND mt2.col1 = mt1.col2)
and mt1.col1 = 3 --here goes the known value
This only works for the data you posted. Please let us know the count for the following scenario:
+----+------------+---------+
| id | col1 | col2 |
+----+------------+---------+
| 1 | 3 | 2 |
| 2 | 2 | 3 |
| 3 | 3 | 2 |
| 4 | 3 | 5 |
| 5 | 4 | 5 |
+----+------------+---------+
Are you looking something like
SELECT (col1+col2) , COUNT(*) AS num_rows
FROM table1
WHERE col1 =3 OR col2 =3
GROUP BY (col1+col2)
HAVING num_rows >1
With this query:
select count(*) count_rows
from table1
where col1=3 or col2=3
group by concat(IF(col1<col2, col1, col2), ',', IF(col1<col2, col2, col1))
you get the result 2.
Replace the 3 from the query in the where clause if there is another input.
The value for concat(IF(col1<col2, col1, col2), ',', IF(col1<col2, col2, col1)) would be "2,3" for records 1-2, (and "4,5" for the third record).